Lighting devices capable of adjusting their emission color are on the market. In these lighting devices, LEDs (light-emitting diodes) are used as a light source, and the light source unit is sometimes modularized (Such light source unit is hereinafter referred to as “LED module.”). As is well known, an arbitrary emission color can be obtained at an arbitrary light emission intensity, by preparing an LED emitting red light, an LED emitting green light, and an LED emitting blue light and adjusting the emission intensity of each of the LEDs.
For natural illumination light, an emission color near the blackbody radiation locus is preferable. In other words, a high color temperature is selected for increasing light brightness, and a low color temperature is selected for dimming light. Color temperature can be varied along the blackbody radiation locus by adjusting the intensity of each two prepared LEDs emitting light at different color temperatures on the blackbody radiation locus (see, for example, Japanese Unexamined Patent Publication (Kokai) No. 2012-113959 (hereinafter referredto as Patent Literature 1)).
FIG. 8 is a circuit diagram of a lighting device (light-emitting device 1) described in Patent Literature 1. As illustrated in FIG. 8, the light-emitting device 1 includes LEDs 2 as a light source, a chromaticity-setting unit 3 for setting a certain chromaticity, and a control unit 4 for dimming the light output from the LEDs 2 to the chromaticity set by the chromaticity-setting unit 3. As the LEDs 2, two types of LEDs 2a and 2b each emitting light of a different chromaticity are used. The LEDs 2a emit light at a lower color temperature, and the LEDs 2b do at a higher color temperature.
The chromaticity-setting unit 3 includes a volume controller 31 which generates color temperature information so that, in dimming operation, when the light output is small, the light-emitting device 1 emits light having a lower color temperature, and the light-emitting device 1 emits light having gradually elevating color temperature as the intensity of the light output increases. The chromaticity-setting unit 3 calculates a chromaticity point on the blackbody radiation locus based on the color temperature information from the volume controller 31 and outputs a duty signal including control information to the control unit 4. The control unit 4 applies a voltage for dimming control to the LEDs 2a and 2b based on the duty signal. The control unit 4 is incorporated in a power supply unit (not illustrated) that turns on the light-emitting device 1.
FIG. 9 is a graph illustrating a light emission characteristic of the light-emitting device 1 illustrated in FIG. 8. In FIG. 9, chromaticity points of the LEDs 2a and 2b are indicated by 2a and 2b. The set color temperature of the LEDs 2a is 2,500 K, and the chromaticity point 2a of the LEDs 2a is on the blackbody radiation locus L. The set color temperature of the LEDs 2b is 5,000 K, and the chromaticity point 2b of the LEDs 2b is positively deviated for both x and y values with respect to coordinates corresponding to the set color temperature on the blackbody radiation locus L. A line segment (2a-2b) on the chromaticity diagram connecting the chromaticity points 2a and 2b in the figure is close to the blackbody radiation locus L. The light emission color of the light-emitting device 1 is determined by a ratio of the light emission amount of the LEDs 2a to that of the LEDs 2b and falls on a point on the line segment (2a-2b).
In FIG. 9, the deviation duv of the chromaticity point 2b (distance from the blackbody radiation locus L) is set to be larger than that of the chromaticity point 2a. The reason is that, when a current increases, the chromaticity point 2b (x and y values of the chromaticity coordinates) of the LEDs 2b is expected to negatively shift like a chromaticity point 2b′. Thus, the emission color of the light-emitting device 1 varies within a range indicated by a broken line W (2a-2b′) in the figure, i.e., within a range more close to the blackbody radiation locus L.
In the light-emitting device 1, an LED module is configured by a light-emitting unit (board 5) in which conductive patterns 41a, 41b, 51a, and 51b are formed on a board 5 and the LEDs 2a and 2b are mounted thereon (see FIG. 8). In other words, Patent Literature 1 can provide an LED module configuring, in combination with the chromaticity-setting unit 3 and the control unit 4, a lighting device (light-emitting device 1) illuminating naturally and comfortably.